Hydrogen is stored conventionally as a gas in steel cylinders at high pressures (e.g., 2,000 psi) and at lower pressures as a liquid in insulated containers. Both methods of storage require comparatively bulky storage containers. In addition to their unwieldy size, such containers are inconvenient due to the high pressure required for gas storage in cylinders and the ever present danger of gaseous hydrogen evolving from boiling-off of the liquid form.
Within recent years, considerable attention has been focused on the storage of hydrogen as a metallic compound, or hydride, of various substances. Metal hydrides can store large amounts of hydrogen at low and even sub-atmospheric pressures in relatively small volumes. This low pressure storage of hydrogen is relatively safe and allows the construction of hydrogen containers having forms significantly different than those presently known.
Hydridable metals are charged with hydrogen by introducing pressurized gaseous hydrogen into valved containers. The hydrogen gas reacts exothermically with the metal to form a compound. Discharging of the metal hydride is accomplished by opening the valve of the container, to permit decomposition of the metal hydride, an endothermic reaction. It has been found expedient when gas is desired from the storage vessel to heat the storage vessel thereby increasing the flow of hydrogen or providing hydrogen at pressures substantially above atmospheric.
During the adsorption/desorption process, the hydridable metal has been found to expand and contract as much as 25% in volume as a result of hydrogen introduction and release from the metal lattice. Such dimensional change leads to fracture of the metal powder particles into finer particles. After several such cycles, the fine powder self-compacts causing inefficient hydrogen transfer. Additionally, and of even greater significance, high stresses due to the compaction of the powder and expansion during hydride formation are directed against the walls of the storage container. The stress within the powder has been observed to accumulate until the yield strength of the container is exceeded whereupon the container plastically deforms, buckles or bulges and eventually ruptures. Such rupture is extremely dangerous since a fine, often pyrophoric powder is violently expelled by a pressurized, flammable hydrogen gas. Small, experimental cylinders of the aforedescribed type have indeed been found to burst when subjected to repetitive charging/discharging conditions.
The problem of expansion and compaction has been recognized in the art to the extent that containers are only partially filled with hydridable metal powders. The problem of hydridable metal powder particle breakdown has been addressed in U.S. Pat. No. 4,036,944 wherein a thermoplastic elastomer binder is used to form pellets of the hydridable metal particles. Although this provides a solution to a portion of the problem of hydrogen storage, polymers are notoriously poor heat conductors, are subjected to thermal deterioration, and may be detrimentally affected by hydrogen. Since heat is generated during hydrogen charging and since heat may, in many cases, be added during discharging, such polymer containing pellets appear to be only partially useful under somewhat restrictive operational conditions.